Compact microstrip quad-band bandpass filter using stub-loaded SIRs

A new compact microstrip quad-band bandpass filter (BPF) using two sets of different stub-loaded stepped impedance resonators (SIRs) is presented. One of the stub-loaded SIRs, which can generate three passbands in the filter design, is formed by loading a short-circuited stub in the center of SIR and two symmetrical open-circuited stepped impedance stubs at two sides. While the other one consists of a simple symmetrical SIR loaded with a short-circuited stub, where a passband with controllable bandwidth can be obtained. The resonance properties of the proposed two stub-loaded SIRs can be both deduced by odd- and even-mode analysis method. For demonstration, a quad-band BPF centered at 2.0, 3.0, 3.9, and 7.2 GHz is designed, fabricated and tested, whose simulated and measured results are in good agreement.